Temperatures must be "just right" for life to develop, astronomers say. / University of Hawaii/University of California

by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

There are likely "tens of billions" of Earth-like planets in our Milky Way galaxy, according to a study released Monday by astronomers from the University of California-Berkeley and the University of Hawaii.

"Planets like our Earth are relatively common throughout the Milky Way galaxy," said astronomer Andrew Howard of the University of Hawaii, who estimates the number at about 40 billion.

In fact, the nearest Earth-like planet may be "only" 12 light years away, which is roughly 72 trillion miles.

In all, about 8.8 billion stars in our galaxy have planets that are nearly the size of Earth and also have a surface temperature conducive to the development of life. But many more stars (those not similar to our sun) also have planets where life could form, which is where the 40 billion-planet figure comes from.

Like Goldilocks tasting the porridge, temperatures must be "just right" for life to develop: Planets must have a so-called "habitable zone" with "lukewarm temperatures, so that water would not be frozen into ice or vaporized into steam but instead remain a liquid, because liquid water is now understood to be the prerequisite for life," said Geoffrey Marcy, a professor of astronomy at Berkeley.

The discovery was based on the most accurate statistical analysis yet of all the observations from the Kepler telescope, a space observatory launched in 2009 specifically designed to locate planets around other stars.

The research was based mainly on an exhaustive, three-year search of Kepler data undertaken by Erik Petigura, a graduate student at the University of California, Berkeley.

"Now, for the first time, humanity has a measure of how common Earth-size planets are around sun-like stars," Marcy added.

Howard says the new estimate of planets means there are 40 billion chances "for life to get started and to evolve."

"The findings are robust, but you have to read the fine print to understand that the numbers are somewhat uncertain," noted MIT astronomer Sara Seager, who was not part of the study. "Overall the result speaks to the growing findings that small planets are everywhere."

"For the past couple of years there has been an emerging consensus that Earth-size planets are common, so in that sense, the result is not hugely surprising," said astronomer David Kipping of the Harvard-Smithsonian Center for Astrophysics, who was also not part of the study. "What is special about this work is the huge effort of the authors to develop a completely independent way of measuring this occurrence rate to that of the Kepler team."

And going beyond our galaxy, Marcy reminds us that the Milky Way is just a typical galaxy within our universe, which contains hundreds of billions of galaxies, each of which has about the same number of sun-like stars as does our Milky Way.

"With tens of billions of Earth-like planets in each galaxy, our entire universe must contain billions of billions of Earth-like planets," Marcy said,

The study was published online Monday in the Proceedings of the National Academy of Sciences using data from the Kepler telescope. The $591 million Kepler telescope is now crippled and nearing the end of its four-year mission.